A puddle of mud dries in the sun and cracks all over. At first the cracks cross at nearly right angles, like streets in a city. But if you wet the mud again and let it dry, the pattern changes: the cracks start meeting in threes, at 120-degree angles, like a honeycomb.
Researchers at Osaka University built a computer model to figure out why. In the model, the mud is a thin layer that shrinks as it dries. The cracks are the places where the layer has been weakened. After rewetting, the cracks "heal," but they leave scars: patches that are no longer as strong.
The cracks are like rivers that straighten their channels with every flood. A river takes the short path, not the winding one. The same goes for cracks: a meeting at a 120-degree angle gives a smaller total length than a meeting at a right angle. So after several cycles of drying and wetting, the cracks rearrange themselves into a more favorable shape.
This knowledge helps predict how drying soils and coatings will crack. In agriculture, for instance, it matters to know when soil will crack after irrigation, and in construction, how long a coating will last if it keeps getting wet and then drying out.
In experiments with coffee paste, after several cycles of wetting and drying the cracks line up at 120-degree angles, like a honeycomb. This is the angle at which three cracks can meet without creating excessive stress.